In two-photon excitation (as a special case of multiphoton excitation), a transition is excited in the excitation structure (term diagram) of a gas, liquid or solid (such as electronic, vibrational or rotational transitions or fine structures) by means of the quasi-simultaneous absorption of two photons of the longer wavelengths xcex1 and xcex2 (where xcex1 and xcex2 can be identical or different), for which purpose an individual photon of the shorter wavelength (xcex1+xcex2)/4 would be necessary otherwise. Two photons in the xe2x80x9clong wavelengthxe2x80x9d (e.g., in the red range) can thus, for example, excite a UV-absorbing transition which normally (i.e., in conventional single-photon excitation) absorbs in the short wavelength (e.g., in the blue range) (FIGS. 1a and 1b). Since two photons are needed to excite a two-photon transition, the transition rate for a given transition depends upon the square of the excitation intensity. For this reason, intensive pulsed laser sources are generally used for two-photon excitation, wherein the two-photon transition probability increases at constant average light output when using shorter but more intensive light pulses.
The first experimental observation of two-photon absorption by Kaiser and Garret in 1961 describes the excitation of a Eu2+-doped CaF2 crystal in the optical region which was only possible after the development of high-power monochromatic ruby lasers. The possibility of two-photon absorption or two-photon stimulated emission was already described in theory in 1931 by Maria Gxc3x6pper-Mayer. The use of the two-photon technique in laser scanning microscopy was first proposed by Denk, Strickler and Webb (1990).
WO 91/07651 describes a two-photon laser scanning microscope with excitation by laser pulses in the sub-picosecond range at excitation wavelengths in the red or infrared range.
EP 666473A1, WO 95/30166, DE 4414940 A1 describe excitations in the picosecond range and above with pulsed or continuous radiation.
A process for the optical excitation of a specimen by means of two-photon excitation is described in DE C2 4331570.
DE 29609850 by the present Applicant describes the coupling of the radiation of short-pulse lasers into a microscope beam path via optical fibers.
At the present time, prober techniques such as OBIC and LIVA are used to detect lattice defects. In OBIC (optical beam induced current), electron hole pairs are generated by means of sufficiently high-energy laser radiation, i.e., photons which can skip over the band gap of the semiconductor being analyzed (i.e., the energy of the radiated photons is greater than the band gap energy EG of the semiconductor; FIG. 2). The locally dependent charge carrier current generated in this way by the scanning laser beam can be utilized for localizing lattice defect locations in the crystal. For this purpose, the wafer to be analyzed is either contacted (prober station) or the wafer is packaged and the technique is applied to the finished integrated circuit. After amplification, this charge carrier current forms the video signal depending on the scanning position (non-optical detection signal). The disadvantage of this method is that generating electron hole pairs in this way is not z-selective. Accordingly, in order to prepare the z-information the wafer must be laboriously polished layer by layer after localizing a defect by means of the two-dimensional technique and must be inspected after every polishing step by means of an electron microscope in order to localize the defect in the z-coordinate as well. LIVA (light induced voltage alteration) is a technique related to the OBIC technique, wherein a constant voltage is applied to the prober electrode (or the IC pin) and voltage changes are detected depending upon the scanning laser beam.
In order to analyze silicon wafers by means of single-photon laser scanning microscopy, a scanning near-infrared laser beam (e.g., Nd:YAG laser at a wavelength of approximately 1064 nm) which is also transmitted to a sufficient extent by doped silicon and can accordingly penetrate deep into the silicon wafer is generally used. In particular, it is possible in this way, in the case of an optically impenetrable metal coating on the upper surface of the IC, to optically penetrate the entire silicon substrate (several mm thickness) with the laser beam from the back (backside imaging or backside OBIC) in order to reach the structured upper side.
It is an object to provide a multiphoton laser scanning microscopy in material analysis, especially in the analysis of structured silicon wafers by means of non-optical detection techniques such as, e.g., OBIC or LIVA. In accordance with the invention, the high localization of the multiphoton excitation in all three spatial coordinates through the use of high-aperture microscope objectives enables nondestructive three-dimensional localization of crystal defects in the semiconductor structures. This technique advantageously dispenses with the detection of lattice defects by means of two-dimensional techniques (e.g., laser scanning microscopy, non-confocally or by detection of non-optical detection signals) and the subsequent required successive mechanical removal of the crystal structure in conjunction with electron microscopy for detecting the defects in the third dimension as well.
In many cases, the concern is with the spatial (x, y, z) resolution of the silicon structure being analyzed in three dimensions. Through the use of excitation light in the NIR(xcex greater than 1100 nm), i.e., beyond the band boundary of silicon, the radiation is transmitted with less absorption through the generally thick (generally doped) silicon substrate. In this case, it is only at the location of the focus formed by the generally high-aperture microscope objective that sufficiently high intensities are achieved for generating electron hole pairs by means of the nonlinear multiphoton excitation process. Accordingly, by means of two-photon microscopy, electron hole pairs can be induced with radiation in the wavelength range of the xe2x80x9coptical windowxe2x80x9d of silicon with extensive z-discrimination.